This invention relates to a video/audio simultaneous transmission system and, more particularly, to a video/audio simultaneous transmission system which can encode a video signal including horizontal sync signal occurring at indefinite time intervals and an audio signals and transmit these encoded signals.
In a video/audio simultaneous transmission system in which a video signal and an accompanying audio signal are transmitted through the same transmission channel, one type has hitherto been used in which each time a horizontal sync signal occurs in the video signal, a special code word for word synchronization is transmitted and encoded audio and video data are successively transmitted subsequent to the special code word in order that even such a video signal of comparatively low sync frequency stability as transmitted from a video apparatus, for instance, a cassette video tape recorder (hereinafter referred to as VTR) may be transmitted. Such a prior art video/audio simultaneous transmission system is disclosed in Japanese Patent Preliminary Publication No. 31277/82.
According to the prior art video/audio simultaneous transmission system, the encoding of the video signal is suspended for a predetermined period in the horizontal retrace period, and during this time period the special code word and encoded audio data are transmitted, whereby even in a case of such a video signal of low sync frequency stability as from a video apparatus, for example, a VTR, the audio signal can be transmitted simultaneously without affecting the video signal at all. Also, the word synchronization can be readily established on the receiving side by extracting the special code word.
In the prior art video/audio simultaneous transmission system, however, in order for the audio signal to be sampled at a predetermined sampling frequency for encoding and simultaneous transmission even when a dropout or large phase jump occurs in the horizontal sync signal of the input video signal, the scale of the hardware must be greatly increased. For example, if a drop-out occurs in the horizontal sync signal in the video signal, the encoded audio data obtained during a plurality of horizontal scanning periods has to be temporarily stored in a buffer memory and successively sent out each time following horizontal sync signals occur. To this end, a buffer memory and a control circuit, these being of considerably large scales, must be added. Therefore, with the prior art video/audio simultaneous transmission system the scale and complexity of the hardware are greatly influenced by the degree of the horizontal sync frequency stability permitted for the input video signal; particularly, where it is ineviable to allow low horizontal sync frequency stability for the input video signal, it is impossible to obtain simple and stable video/audio simultaneous transmission.